A mixer or frequency mixer or frequency conversion device is an electrical circuit that creates new frequencies from frequencies of signals applied to it. In its most common application, two signals at frequencies are applied to a mixer, and it produces new signals as the sum or difference of the original frequencies. Other frequency components may also be produced in a practical frequency mixer. Mixers are widely used to shift signals from one frequency range to another, but also as a product detector, modulator, phase detector or frequency multiplier. Mixers can be coupled with a filter to either up-convert or down-convert an input signal frequency, but in receiver systems they are more commonly used to down-convert an input signal, known as radio frequency (RF) signal, to a lower frequency, known as the intermediate frequency (IF), to allow for a simpler receiver design. A side effect of using mixers is that in addition to generating their own noise and converting frequencies of signals and noise in the desirable input frequency band, they also frequency-convert noise in other harmonic side bands (HSBs), which all appear at the mixer output. Non-linear circuits also react to signals in their harmonic side bands and therefore can be considered as a special subset of mixers. Mixers can be made with any non-linear circuits. For example, for noise-parameter-measurement purposes, a non-linear power amplifier can be considered as a mixer with the input signal, local oscillator (LO) signal, and the output signal being at the same frequency.
In systems where mixer noise affects the overall sensitivity, an understanding of how to minimize noise of a given mixer can be important. Noise figures of mixers are generally poor due to noise from HSB frequencies being converted to the output IF. Several noise-parameter measurement techniques have been developed for linear two-port networks, such as low noise amplifiers (LNAs), transistors, etc., but not for frequency conversion devices. Measurement of mixer noise parameters is becoming important as there are now some circuits, which avoid the use of low noise amplifiers (LNA) in front of mixers to save power, reduce design complexity, and/or lower costs. Using a mixer in front of a receiver down converts noise from various HSBs to its output. This makes signal detection more difficult. In addition, noise at the output of non-linear circuits, such as power amplifiers, couples to inputs of nearby receivers thereby reducing their sensitivity.
Currently, most studies on mixer noise are from the circuit design perspective, where noise figure (NF) of a mixer is modeled in terms of physical circuit parameters. These methods do not fully characterize the noise contribution of HSBs, and in particular the image frequency (IM) band, to the overall mixer noise figure and do not fully model the sensitivity to mismatch between the optimum admittance (or equivalent impedance or reflection coefficient) and the actual implemented admittance at the HBS frequencies. The previous art fixes the IM port with a fixed admittance Yi. Fixing this admittance effectively reduces the mixer to a two-port model where effective two-port noise parameters are solved for. In general these effective two-port noise parameters are dependent on a fixed admittance Yi. Thus, the noise model is not complete as effects of all impedance terminations on NF are not characterized.
Another approach to modeling the noise of a mixer is done computationally using methods such as combining a conventional time-domain model with circuit noise information; obtaining an equivalent circuit model for transistors to generate a bias-dependent model, the bias-dependent model being used as a seed to obtain noise values for a large signal model; and lastly, applying an impedance field method to high and low frequency microscopic noise processes. The total noise power contributed at the intermediate frequency (IF) output of the mixer from each such HSB s, each of which is represented with an equivalent input port, is represented by a correlation matrix of equivalent spectral noise currents and/or voltages at several different frequencies and bias points of transistors under large signal operation. These methods are ideal for the computation of mixer noise correlation coefficients in a simulated environment; however, no method exists to measure the corresponding mixer noise parameters or noise correlation coefficients for each port.
A need therefore exists for a method and system that allows for the modeling and, more importantly, for measuring the noise behavior of each HSB in order to determine circuit operating conditions that minimize the noise of systems including mixers. A need exists for a method to predict system level noise performance, which is better than conventional techniques. A need exists for a method to determine how sensitive the NF of the frequency conversion device is to LO phase noise. A need exists to minimize noise contribution of each HSB to the output of the frequency conversion device by matching HSB to optimal impedances. A need exists for a method to calculate the tradeoff between input power matching and NF. If a LNA is not being used, a need exists for a method to design antennas with better system signal to noise ratio. A need exists to fully model the sensitivity mismatch between the optimum admittance and the actual implemented admittance at HSB frequencies. A need exists for a method that allows for the full characterization of all HSB frequencies, modelled by corresponding HSB ports, by giving each port its own set of noise parameters. A need exists to determine optimum noise matching for non-linear devices.